Hi  Olumuyiwa.

The close on due date feature is independent of the suspended user feature.

Suspended Users

This talks to 2 separate things.

If the suspended user is the subject of an activity, then these can be closed and hidden, along with all the (associated) participant instances.  Because as u say, the user has gone. 

If the suspended user is a participant whether their responses are hidden depends of whether they have responded.  We will retain anything they have added (because what a suspended user did say while they were active is relevant to a conversation.  If they haven't replied their empty response instance will be hidden (because they aren't going to respond).

Close on due date and optionally hide empty responses 

These features are designed to close and activity without having to wait for all participants to respond (the activity is 'complete').  E.g.  I want feedback from my direct reports , but I don't need all my reports to respond, that could take forever.  At the due date close the activity and hide the empty responses.
Arguably an improvement to this idea is that a setting is added to allow activities that have been 'closed down' to also be deemed 'completed' so the activity's state is 'we're done'.

Does this make sense?